In typical boiling water reactors used for power generating operations, reactor coolant is circulated endlessly around a flow path comprised of a core entrance plenum located below the heat producing fuel core, thence through the core itself, thence through an upper plenum region located above the core and which serves to collect all the coolant passing through fuel assemblies comprising the core, then on through an assembly of steam separators overhead of the core upper plenum, and finally back downward outside the core, along a region termed the downcomer annulus, to return to the core lower plenum. If the reactor is designed as a natural circulation type boiling water reactor, this final flow path outside the core is direct and uninterrupted. A cylindrical member enshrouding the core and extending some distance both above and below the elevations containing the core is positioned between upwardly flowing coolant passing through the reactor core, and downwardly flowing coolant recycling back to the core lower plenum. If the reactor type is a forced circulation reactor, some form of pumping mechanisms are positioned outboard of the core shroud member along this portion of the flow path to amplify the pressure head otherwise present in the core lower plenum region.
The water coolant in such boiling water reactors during their power generating operation exists, at the core entrance, in the form of a subcooled liquid. This subcooled liquid has been produced by mixing, early along the downcomer annulus flow path, two streams: a feedwater stream that has large temperature subcooling relative to reactor operating pressure conditions, and a saturated liquid stream which has been derived by the partitioning, via the assembly of steam separators aided by steam dryers, of two-phase steam-water mixture produced at the exit from the core. The feedwater stream has a mass flow rate that is controlled to match the reactor steam output mass flow rate, so that the coolant inventory and water level within the reactor remain nominally constant. The aforementioned partitioned saturated liquid stream typically has a mass flow rate many times the mass flow rate of the feedwater stream, so that the temperature of the mixed stream arriving in the core lower plenum lies closer to the coolant saturated conditions than to the feedwater entrance conditions.
As the reactor coolant passes through the core, heat is transferred from the fuel assemblies to the coolant. The water coolant emerges from the heat producing fuel core as a two-phase mixture of steam and water, the proportions of which vary depending on such factors as the power output from the fuel, the amount of subcooling present in the feedwater, the total hydrodynamic flow resistance presented by the fuel core design and structure and its wetted surface, and the amount of orificing representing restrictions to flow immediately prior to the entrance of the coolant into the individual core fuel assemblies.
Conventional fuel assemblies of boiling water reactors are composed of a multiplicity of fuel units, such as rods, grouped together in bundles, with each bundle surrounded by an open ended channel for flow lengthwise therethrough. These channeled bundles of fuel units are spaced apart from each other to provide intermediate spaces for insertion of control blades. Thus, there are ample areas for coolant water bypass flow beyond close proximity to the heat producing fuel units within a bundle.
Bypass flow coolant water passes through the fuel core without closely encountering the high energy emanating from the fuel and enters the core upper plenum consisting substantially of saturated liquid with perhaps a small amount of steam. This bypass effluent joins the two-phase steam-water mixtures exiting from individual fuel assemblies comprising the core. These two effluents rapidly mix together within the core upper plenum losing identity from their origin, with the result of a combined overall steam-water mixture containing significant proportions of water.
Typical boiling water reactors utilize mechanical steam separators to separate steam from the steam-water mixture leaving the fuel core. Some early reactor designs employed free-surface steam separation means where steam separates unaided from the free-surface, and saturated water remains in the bulk coolant which is recirculated back around through the fuel core. This means of steam separation is feasible as long as the steam-leaving velocity--the bulk average velocity of the steam taken across the available pathway flow area--is no greater than about 1.8 feet per second. If steam-leaving velocities become greater than this limiting value, the steam tends to carry along an unacceptably high moisture content. The high moisture levels saturate the moisture-drying capability of the steam dryer whereby there is an excessively high moisture content in the steam leaving the reactor and supplied to a turbine or other steam utilizing mechanism. Such high moisture contents in steam tend to accelerate corrosion/erosion of the turbine blades and other components.
Free-surface separation capabilities can be achieved if the reactor pressure vessel cross-sectional area is made large enough. Cost economics, however, often dictate that minimum diameter pressure vessel be used whereby mechanical steam separators have been developed and employed to handle the higher power output steam production levels of various current boiling water reactor designs. In these later reactor designs the steam bulk average velocity moving through the wet steam plenum region immediately downstream of the mechanical steam separators is about 5 feet per second.
The steam exit qualities tend to be higher from the central region of the fuel core than from the peripheral region of the fuel core. However, it is desirable that the flow rates and the steam-water mixture proportions entering the steam separator standpipes from the core upper plenum be relatively uniform. To facilitate achieving more uniform steam-water mixtures for entry into the standpipes of the steam-water separators above the fuel core and core upper plenum, the standpipe entrances typically are separated from the fuel assemblies by a distance of at least about 5 feet. Turbulent mixing occurring between the fluid plumes leaving adjacent fuel groups of the core, each with a different void content, comprises one mechanism acting to produce more uniform steam-water mixtures adjacent to the steam separator standpipe entrances. More significant with respect to achieving uniformity of flow mixture, is the hydrodynamic flow resistance represented by the standpipes each with their end-mounted steam separators. Complete flow mixture uniformity entering the steam separator standpipes is at best difficult to achieve and, even with a five foot separation between the fuel core assembly exits and the separator standpipe entrances, is not a design basis used for reactor performance evaluations.
A conventional boiling water reactor steam separator assembly consists of a domed or flat-head plate topping the core upper plenum which is superimposed over the fuel core. An array of steam separator standpipes are affixed such as by welding to the core upper plenum top plate with the standpipes in fluid communication with the interior of the core upper plenum. A mechanical steam separator, such as a three stage centrifugal axial flow separator, is mounted on the other and upper end of the standpipe affixed to the top plate.
One function of the standpipes is to provide a standoff separation of the larger-diameter steam separators, which are generally arranged in a relatively tightly compacted arrangement in which external diameters of adjacent separators are almost in contact with each other, whereby separated liquid coolant discharged from the bottom of the separator has a more open flow-path outward from the reactor longitudinal axis and out to the downcomer annulus region which lies at the inboard periphery of the reactor pressure vessel. A second reason for the standpipes on a high power output natural circulation reactor using mechanical steam separators is to provide a natural circulation "chimney" of two-phase (and thus low-density) coolant water wherein the chimney height provides part of the natural-circulation driving head for coolant water flow circulation within the reactor.
The steam separator assembly is supported by a flange at the top of the core shroud. The flange joint between the steam separator assembly and the core shroud is a metal-to-metal contact and does not require a gasket or other sealing devices requiring service or replacement. Moreover, the fixed axial flow type steam separators are constructed of stainless steel and have no moving parts whereby they are maintenance free.
In each separator, the steam-water mixture rises up from the core upper plenum through the standpipe into the separator unit where it impinges upon helical vanes that give the steam-water mixture a spinning movement establishing a vortex whereby centrifugal forces separate the denser water from the steam in several successive stages. Steam leaves the separator at the top and then passes out into the wet steam plenum located immediately below the steam dryer.
The separator water exits from the lower end of each stage of the separator and enters the underlying pool that surrounds the standpipes joining the downcomer annulus flow of reactor subcooled coolant. The steam exits from the separators can either be all in the same horizontal plane, or the separator units can be arranged with their tops in a convex crown with a higher center to compensate for a convex crowned water gradient to the underlying pool surrounding the standpipes.
The mechanical separator has certain principal performance requirements; namely, over a range of approximately 30 inches of water level variation about about the midplane of the separator units housing, and over a range of reactor power operating conditions from about 25% up to and slightly exceeding 100% the steam separator is required to deliver wet steam into the wet steam plenum with moisture contents generally not to exceed 10% by weight of the wet steam effluent, and to deliver water out the bottom of the stages of the unit stripped of steam to the extent that bulk average steam carryunder generally does not exceed 0.25% by weight of effluent.
The nominal volumetric envelope of the steam separator assembly is defined by the horizontal plane of its lower terminal that contacts the top of the core shroud, its peripheral sides that provide part of the five foot standoff from the fuel core assembly exits, the circumscribed diameter of the outermost row of the standpipes, and the generally horizontal plane of the exits to the steam separators.
At rated operated conditions with such mechanical steam separators the pressure drop from the core upper plenum to the wet steam plenum at the separator exit below the overhead steam dryer assembly is about 6.7 pounds per square inch of irreversible head losses (friction, form drag and exhaust losses).
Irreversible head losses anywhere along the reactor coolant circulation path are penalizing to the efficiency of operation of the reactor pressure vessel and/or the overall nuclear steam supply system. For natural circulation boiling water reactors, irreversible head loss means greater chimney height has to be provided, which means the reactor pressure vessel probably has to be made larger, namely taller, and therefore more costly. Also the reactor containment building and probably other components would need be made larger contributing to greater costs.
In forced circulation boiling water reactors irreversible head loss would require greater pumping power to accomplish core recirculation flow, and in turn greater capital and greater operational costs for the pumping system and thus poorer net plant heat rate.